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Programmers' FAQ
http://www.aha.ru/~kis/progfaq.htm
A small series 3a/SIBO programming FAQ.
Psion OPL Programming Reference
http://dialspace.dial.pipex.com/lordjohn/oplref/
From the Psion Series 3a distribution.
Twiddlebit Software
http://www.twiddlebit.com/
Twiddlebit Software: OPL development tools and OPXs (OPP, OPPDBG, GPRINTER.OPX)